In ue_attach_post_task(), initialize curvnet to vnet0 before calling if_attach().
Before this patch, curvnet was NULL.
When the VIMAGE kernel option is enabled, this eliminates
kernel panics when USB ethernet devices are plugged in.

PR: 183835
Submitted by: Hiroo Oono <hiroo.ono at gmail dot com>

(cherry picked from commit 3e3bcb42369fce6f32aa711d1cb579ee8f374b37)